Transmission and Distribution Systems
Overview
Transmission and Distribution Systems form the backbone of electrical power delivery from generating stations to consumers. This chapter covers the journey of electricity through extra-high voltage transmission lines, substations, and distribution networks. For ITI Electrician Semester 4 (NCVT), key focus areas include voltage levels, types of lines, insulators, cables, feeders, distributors, and service mains — all frequently tested in theory and practical papers.
Typed Notes
Transmission System — Voltage Levels and Lines
Electric power generated at 11 kV or 33 kV at power stations is stepped up by a step-up transformer to 132 kV, 220 kV, 400 kV, or 765 kV for long-distance transmission. High voltage reduces current, which reduces I²R losses. Overhead lines use ACSR conductors — aluminium carries current while the steel core provides mechanical strength. The sag in conductors is intentionally kept to allow for thermal expansion. Transmission lines above 66 kV are called Extra High Voltage (EHV) lines. Ground wires (earth wires) on towers protect against lightning strikes.
Insulators — Types, Uses, and String Efficiency
Insulators support conductors and prevent leakage of current to the tower/pole. Pin-type insulators are used up to 33 kV on straight runs. Suspension (disc/string) insulators are used above 33 kV; multiple discs are connected in series — this is called a 'string'. Strain insulators are used at dead ends, corners, and sharp angles to withstand mechanical tension. Each disc in a string is rated at 11 kV. String efficiency = (Voltage across string) ÷ (n × Voltage across bottom disc) × 100%. Guard rings improve string efficiency by equalising voltage distribution.
Distribution System — Feeders, Distributors, and Service Mains
The distribution system operates at 11 kV (primary) and 415 V / 230 V (secondary/LT). A Feeder is a conductor that carries power from substation to distribution points — no intermediate tapping is done, so current is constant throughout. A Distributor has tapping points at various intervals for consumers; current varies along its length. Service mains are small cables connecting the distributor to the consumer's meter. Distribution can be radial (simple, less reliable) or ring main (loop, more reliable). DC distribution uses 3-wire system; AC distribution uses 3-phase 4-wire system for balanced loads.
Underground Cables — Construction and Grading
Underground cables are used in urban areas where overhead lines are not feasible. A cable consists of: (1) Conductor — stranded copper or aluminium, (2) Insulation — XLPE or paper impregnated with oil, (3) Metallic sheath — lead or aluminium to prevent moisture, (4) Armour — steel tape/wire for mechanical protection, (5) Outer serving — jute or PVC for corrosion protection. Cables are classified by voltage: LT cables (up to 1 kV), HT cables (up to 11 kV), EHV cables (above 66 kV). Capacitance of underground cables is much higher than overhead lines — this is a key exam point.
Topics & Sub-topics
Har topic ke saath sub-topics aur example — chapter ka complete coverage.
01Voltage Levels
- Generation: 11 kV / 21 kV
- Transmission: 132/220/400/765 kV
- Distribution: 11 kV → 415/230 V
Step-up se line loss kam hota hai — isliye 400 kV par door tak bhejte hain.
02Overhead Line Components
- Conductors: ACSR, AAAC
- Insulators: pin, suspension, strain
- Poles, towers, cross-arms
ACSR = aluminium conductor steel reinforced — strength + conductivity dono.
03Substations
- Step-up / primary / secondary
- Transformer + breaker + isolator
- Indoor vs outdoor
Colony ke paas wala 11 kV/415 V transformer — secondary substation.
04Distribution System
- Feeder → distributor → service mains
- Radial vs ring main
- LT boxes, pillar boxes
Ring main distribution mein fault par do taraf se supply — outage kam.
Key Concepts
Revision ke liye ek nazar — exam se pehle yahi padhna hai.
- 01Transmission voltage in India: 132 kV, 220 kV, 400 kV, 765 kV (EHV); distribution voltage: 11 kV (primary) and 415 V / 230 V (secondary/LT)
- 02Overhead transmission lines use ACSR (Aluminium Conductor Steel Reinforced) conductors for high strength and low cost
- 03Three types of insulators: Pin type (up to 33 kV), Suspension/String type (above 33 kV), Strain type (at dead ends and sharp turns)
- 04Distribution system components: Feeder (carries bulk power, no tapping), Distributor (tapping points for consumers), Service mains (final connection to consumer)
- 05Power loss in transmission line: P = I²R; to reduce loss, voltage is stepped UP for transmission and stepped DOWN for distribution using transformers
Questions & Solutions
Pehle khud try karo — phir solution kholo. Yahi tariqa topper banata hai.
Why is electric power transmitted at high voltage over long distances?
What is ACSR conductor? Why is it preferred for overhead transmission lines?
Differentiate between a Feeder and a Distributor in a distribution system.
What is string efficiency of a suspension insulator string? How can it be improved?
Name the layers of an underground cable from inside to outside and state the purpose of armour.
Transmission lines carry power at EHV (132 kV–765 kV) using ACSR conductors to minimise I²R losses. Insulators (pin, suspension, strain) support and isolate conductors. Distribution at 11 kV (primary) and 415/230 V (secondary) uses feeders, distributors, and service mains. Underground cables have higher capacitance than overhead lines. Ring main distribution is more reliable than radial. These concepts are core to NCVT ITI Electrician Semester 4 exams.
Chapter-end quiz
4 MCQsOption tap karo — turant pata chalega sahi ya galat, explanation ke saath.
Q1.Which type of insulator is used at the dead end of a transmission line?
Q2.In a distribution system, the voltage of the secondary (LT) distribution network in India is:
Q3.The full form of ACSR conductor used in overhead transmission lines is:
Q4.Compared to overhead lines, underground cables have:
Mark karo — dashboard par progress dikhega.